SBW gone and Aussies teeter on Rio edge

Japan again slayed a Goliath, Australia are teetering on the precipice without their playmaker and SBW's Rio Games ended after just nine minutes.

If drama is any measure, then rugby sevens assured its place on the Olympic program for decades to come on Tuesday.

An astonishing day of upsets, big-name injuries and breathtaking attacking play has left nine teams all scrambling for eight quarter-final positions in their final pool matches on Wednesday.

It's debatable what was the biggest news story: world No.15-ranked Japan producing their first defeat of gold medal contender New Zealand or Sonny Bill Williams' ruptured Achilles which will sideline the superstar for at least six months.

Williams sacrificed the Super Rugby season and three All Black Tests for his Olympic dream but it was all over shortly after halftime in NZ's shock 14-12 defeat.

For once, New Zealand's highest-profile Olympian - who has won two Rugby World Cups, two NRL titles and Super Rugby championships - hasn't achieved what he set out for.

"It's devastating for Sonny, to lose him so early in the tournament," said coach Sir Gordon Tietjens, who tipped Williams to be back playing sevens again.

"He's a great professional. He's very infectious, he certainly helps the younger players, he's got a great work ethic and it's such a shame because he got to understand the game of sevens so well over the world series."

The All Black sevens also lost Joe Webber to a dislocated shoulder in the bewildering loss to an inspired Japanese outfit and must survive the rest of the tournament with 11 players after calling in Sione Molia over Liam Messam.

Just like the Brave Blossoms' opening match World Cup heroics against South Africa last September, Japan played like men possessed to bring down the 12-time world series champions.

"I'm still shocked to be honest," said NZ-born playmaker Lomano Lemeki after his adopted nation desperately defended their line after the siren to win.

"That was crazy. Everyone was flying just like a kamikaze plane trying to chop their legs off."

Japan almost bagged a second big scalp but Great Britain hung on for a thrilling 21-19 win which means the minnows could deny Australia a quarter-final place by beating Kenya on Wednesday.

World Rugby boss Brett Gosper told AAP the result was a massive boon for the sport's already rosy Olympic future.

The Australians were a first day disappointment, wilting badly against France 31-14 before improving slightly to stay alive with a 26-12 victory over Spain.

They must upset in-form South Africa to be a strong chance of progressing but have an uphill task with five-eighth Lewis Holland ruled out after straining his hamstring against Spain.

"He's a class act in that 10 position but if he's not right, he's not right," coach Andy Friend.

For-and-against is likely to be the deciding factor in which eight teams go through with a loss almost certain to mean the end for Australia.